A new generation of jackup rig, known as the cantilevered type rig, is increasingly being used in offshore drilling of oil wells. In this new generation of oil rigs, a platform is supported from the sea floor by a plurality of legs which normally are jacked relative to the platform until engagement with the sea floor. A main deck is provided by the platform and drilling is performed from a derrick which is positioned in a cantilevered position to one side of the platform. The derrick is movable on skids which permit the derrick to be skidded out in its extended cantilevered position for drilling to one side of the rig platform. In this way, no openings in the platform or main deck are required to permit the drilling operation to be conducted beneath the derrick. When drilling is completed or when the platform is in transit to the drill site, the derrick is skidded in or retracted onto the main deck.
It is also the practice in the use of these new generation rigs to skid in or retract the drilling derrick during severe weather conditions. This precaution is taken to position the derrick in a more secure position and to provide protection from severe storms and ocean activity. In these situations where the derrick is skidded in and drilling is in progress, the wellhead and casing extending above the sea floor are supported by "mudline suspension". In this situation, the exposed wellhead and uppermost casing sections are supported from the floor of the sea. Where the water depth is 100 to 150 feet or greater, this support is insufficient, particularly during storm conditions.
In a conventional (not-cantilever type) drilling unit, some support is provided to the wellhead and the uppermost casing sections by use of support cables attached between the drilling platform and the wellhead. This type of support cannot be provided on a cantilever type rig when the derrick is skidded in over the platform. As a result, wellheads and uppermost casing sections have been lost during storm conditions. Where such damage occurs, substantial losses in man hours, equipment and drilling down time are suffered, and a severe blowout hazard arises.